An organic light emitting diode (OLED) is an active light emitting device driven by a current. Due to its unique characteristics of a self-light emitting, a quick response, a wide angle of view and being manufaturable on a flexible substrate and the like, an organic light emitting display based on the OLED is predicted to become a mainstream of the display field over the next few years.
Each display unit in the organic light emitting display is composed of the OLED. The organic light emitting display may be divided into an active organic light emitting display and a passive organic light emitting display according to their driving modes, wherein the active organic light emitting display refers to that, for each OLED, a current flowing through the OLED is controlled by a thin film transistor (TFT) circuit, and the OLED and the TFT circuit for driving the OLED are composed of a pixel circuit.
A typical pixel circuit is as shown in FIG. 1, comprising two TFT transistors, one capacitor and one OLED, wherein a switching transistor T2 transmits voltage on a data line to a gate of a driving transistor T1, and the driving transistor T1 in turn converts this data voltage into a corresponding current to be supplied to the OLED device. The corresponding current can be expressed as an Equation as follows:
                                                                        I                OLED                            =                            ⁢                                                1                  2                                ⁢                                                      μ                    n                                    ·                  Cox                  ·                                      W                    L                                    ·                                                            (                                              Vgs                        -                        Vth                                            )                                        2                                                                                                                          =                            ⁢                                                1                  2                                ⁢                                                      μ                    n                                    ·                  Cox                  ·                                      W                    L                                    ·                                                            (                                              Vdata                        -                        Voled                        -                        Vth                                            )                                        2                                                                                                          (        1        )            
Wherein Vgs is a potential difference between a gate and a source of the driving transistor T1, μn is a carrier mobility, Cox is a capacitance of an insulation layer of the gate, W/L is a width-length ratio of the transistor, Vdata is a data voltage, Voled is an operating voltage on the OLED, and Vth is a threshold voltage of the driving transistor T1. It can be known from the Equation (1) that: if the Vths are different among different pixel units or the Vth drifts as time lapses, then there are variances in the currents flowing through the OLEDs, thus influencing a display effect. In addition, when the operating voltages of the OLEDs are different due to a non-uniformity of the OLED devices, variances in the currents may be occurred too.
At present, there are many kinds of pixel circuits for compensating the variances in the currents caused by the non-uniformity, the drift of the threshold voltages Vth and the non-uniformity of the OLEDs, but these kinds of pixel circuits are generally realized by disposing the drive TFT in a manner of a diode connection as shown in FIG. 2, but such structure is only applicable to an enhancement type TFT. For a depletion type TFT, it may still be turned on in a case of Vgs=0, therefore voltages stored in the TFT do not comprise any information on the threshold voltage Vth. As a result, for the depletion type TFT, the existing pixel circuits are unable to compensate the variances in the currents caused by the non-uniformity of the threshold voltage.